An apparatus for measuring the degree of beating of a fibrous suspension is described in our Swedish Pat. No. 351,927 which corresponds to U.S. Pat. No. 3,688,563. This apparatus may be connected to a conduit for flowing pulp to render possible a periodical withdrawal of samples of pulp and an accurate determination of their beating degree. If desired, the result of measuring may be obtained in .degree. SR.
The apparatus comprises a vertical sampling cyclinder closed at the bottom end and open at the upper end, and having its upper end connected to a corresponding opening in the lower wall of a substantially horizontal portion of said conduit. A cover plate is mounted above the cylinder so that, when in use, it is disposed within the conduit, the cover plate being movable to and from a position in which the open upper end of the cylinder is kept closed. A piston movable in the cylinder is composed of a strainer disc or plate and a solid disc normally covering the lower side of the strainer disc but movable therefrom to form a space below the strainer disc when the latter reaches its lowest position. A tapping pipe provided with a valve extends from said space to a measuring vessel. A valve controlled fluid pressure conduit is connected to the upper end of the cylinder, whereby when, in use, the apparatus is connected to the conduit, a sample of pulp filling the cylinder may be dewatered under pressure when the upper end of the cylinder has been closed by the cover plate. The amount of water removed from the sample of pulp, which is collected in the measuring vessel, may be used as a measure of the degree of beating of the pulp.
Irrespective of the choice of pressure fluid (gas or liquid), it is of essential importance for a reliable result that the dewatering of the sample of pulp through the strainer disc occurs at a constant pressure. If a liquid is chosen only water under pressure has been found to be useful in practice and in such case it has proved to be of less importance if some of the pressurized water supplied from above in the sampling cylinder mixes with the upper layer of the pulp sample. The explanation thereof may be that a filtering fiber cake is rapidly formed on the strainer disc at the very beginning of the straining operation, and the permeability of the fiber cake for water is then practically independent of the dilution of the filtered suspension. Thus, in this case, the ability of the pulp sample to permit the passage of water is measured.
In laboratory experiments using a transparent cylinder it has now become evident that temporary interferences appear in withdrawal of a sample of pulp from a pulp flowing through a conduit, said interferences result in an uneven distribution of the fiber cake and on certain occasions even direct passage may occur, which, in turn results in incorrect results. The measuring result is also affected if the strainer disc does not completely tighten against the cylinder wall when said disc is in its lowest position.